Here's a short series of videos from Microphone manufacturers Sennheiser that will help you improve your sound when shooting
The most important lesson? Get it right at the time. You can't get extraneous sound out of your recording once it's done - unless you have software like this. But even so, its worth spending time and effort to get your recordings absolutely clean.
As you'll hear in one of the videos, bad audio can actually make people think that the images are bad as well!
Thanks to Geert Verdickt for putting together a great series of tutorials.